Web.config modification in multi server farm

i have a Sharepoint web app which has web.config modification.when i do it in single server farm, it works very fine but when i do the same thing in multi server farm which has 10 servers(2 WFE,2 APP and 6 search), it doesnt work as expected.

<modules>
<add name="Session" type="System.Web.SessionState.SessionStateModule" />
</modules>

This is the modification i do. please guide me